1. Field of the Invention
The present invention relates to a car security system and, more particularly, to a car security system which is capable of detecting, using a proximity sensor such as an infrared sensor or a radar sensor, approach of any unauthorized person to a vehicle.
2. Description of the Related Art
A car security system is known (see FIG. 14) which is capable of detecting the approach of an unauthorized person to a vehicle by using a radar sensor. The radar sensor is disposed in, for example, a vehicle compartment and irradiates high-frequency electromagnetic waves therearound. Any motion of a person produces a low-frequency noise in the electromagnetic wave and, when the level of the noise component exceeds a predetermined level, a detection signal is generated which indicates presence of the person. The output of the radar sensor is input to the main part of a car security system which is installed on a predetermined portion of the vehicle. The main part of the car security system is set to the armed state in response to an arming operation through a transmitter handset by the driver after getting out of the vehicle, so as to monitor the output of the radar sensor to detect any abnormality. When a detection signal is issued by the radar sensor, the main part of the security system decides that an abnormal situation has occurred and activates an alarming function such as flickering of headlights or generation of an alarm sound by a buzzer or a siren so as to give a warning to the unauthorized person approaching the vehicle or to make such a person run away.
This known car security system has the following drawback due to the fact that the monitoring area is fixed. For instance, when the vehicle is parked at a position near a tree with the monitoring function set to cover the space around the vehicle to enable quick detection of an unauthorized person approaching the vehicle, the radar sensor tends to generate a detection signal upon sensing movement of the twigs of the tree. In such a case, the main part of the car security system erroneously operates to decide that an abnormal situation has occurred and wrongly produces alarm, even though no person is approaching the vehicle.